The present invention relates to an electronic device for the control of electromagnetic self-braking current in reversible rotating electric commutator machines, in particular for electric machines with series excitation, generally indicated as xe2x80x9cUniversal Motorxe2x80x9d.
It is known that electric rotating commutator machines can be made to function as generators, even if only for a limited time, that is they can invert from electric motors to generators.
This method of functioning is considerably interesting for various types of industrial applications and amongst these the most interesting is the possibility of creating controlled electromagnetic self-braking in motors for electric tools.
Nevertheless, when the operation of functioning reversal from motor to generator in short circuit is carried out, a rapid current transient is created, lasting a few tenths of a millisecond, which however reaches peak values of intensity in the order of hundreds of Amperes. Such a high value causes irreversible damage to the components of the electric machine and in particular the commutator, the brushes and the windings suffer irrevocable damage.
In view of the state of the art described, the object of the present invention is to make an electronic device which can control the current circulating in the electric machine windings and keep the value of said current constant for the whole period needed for the machine to brake.
According to the present invention, such object is reached by means of an electronic device for the control of the self-braking current in reversible rotating electric commutator machines, fitted with an inductor winding and an armature winding, including reversing means for controlling the functioning of said electric machine as a generator by mutual reversal of the polarity of said inductor winding and said armature winding, characterised in that it includes a switchable current control circuit and an excitation circuit, distinct from each other, said control circuit including an electric load, a power device for connecting and disconnecting said electric load in series in the input circuit of said inductor winding to dissipate the power produced by said electric machine when it functions as a generator, and a variable duration pulse generator for controlling the switching of said power device, and said excitation circuit including voltage generator means for generating, at the time of reversing the machine to function as a generator, a pre-set sign voltage to cause the self-excitation of the machine for its functioning as a generator and, in consequence to said self-excitation, the activation of said variable duration pulse generator for switching said power device to the conditions of connection and disconnection of said electric load.
Thanks to the present invention a device for the control of the electromagnetic self-braking current in reversible rotating electric commutator machines can be made, which, through the use of a pilotable switchable control circuit which connects and disconnects an electric load, such as a resistance or other device, in series with the inductor winding, dissipates in heat all or nearly all the power produced by the machine when functioning as a generator. The current in the circuit including the inductor winding and the armature winding is thus kept constant and causes no damage to the machine functioning as a generator. In addition, said constant current creates in consequence a constant electromagnetic torque on the rotor of the machine.
It must also be noted that the technique of controlling the current with a switching power device in parallel with the electric load permits transfers of very high power with reduced dimension, reasonably priced and high efficiency electronics, that is with quite limited thermal dispersion.
Another important effect of the use of a switchable control circuit consists in the limited consumption in stand-by condition, which, with adequate dimensioning of the components, permits the functioning capacities of the control circuit to be kept for several seconds after the disconnection from the mains supply.
The excitation circuit activates the self-excitation of the machine and then the switching of the power device included in the current control circuit for the connection and disconnection of the electric load in series with the inductor winding. Said excitation circuit uses for this purpose an electric voltage pulse generated to enable the self-excitation of the machine and, with it, the activation of the variable duration pulse generator for switching the power device to electric load connection and disconnection conditions.
In substance the excitation circuit carries out the theoretic function of a generator of pulse voltage with adequate internal impedance. The pulse produced by this generator starts at the time the current reverser of the electric machine is commutated and lasts for several milliseconds, after which the generator excludes itself.
The control device according to the invention has turned out to be particularly suitable for electric machine that can be reversed from motor to generator. Its use is also foreseeable in different contexts, for example for controlled electromagnetic brakes.